Vacuum retention method and superconducting machine with vacuum retention

ABSTRACT

A superconducting machine includes a superconductive device and a vacuum enclosure containing and thermally insulating the superconductive device. A cold-trap is configured to condense gases generated within the vacuum enclosure, and a coolant circulation system is adapted to force flow of a cryogen to and from the superconductive device and the cold-trap. A cryogenic cooling system is configured to cool the cryogen in the coolant circulation system upstream of the superconductive device. A vacuum retention method, for a high-temperature superconductive HTS device, includes applying vacuum to the HTS device to thermally insulate the HTS device, condensing gases generated around the HTS device using a cold-trap, flowing a cryogen to and from the HTS device, and flowing the cryogen to and from the cold-trap.

This application is a division of application Ser. No. 10/331,059, filedDec. 27, 2002, now U.S. Pat. No. 6,708,503, which is hereby incorporatedby reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates generally to vacuum retention for asuperconductive machine and, more particularly, to cryogenic cooling andvacuum retention for a high temperature superconductive (HTS) machine.As used here, the term “cryogenic” refers to a temperature less thanabout 150° Kelvin.

One exemplary superconductive machine is a superconductive rotor forelectric generators and motors. Other superconductive machines includemagnetic resonance imaging (MRI) systems, for medical applications, andmagnetic levitation devices, for transportation applications. Typically,a superconductive coil assembly of a superconducting magnet for asuperconductive device includes one or more superconductive coils woundfrom superconductive wire and which may be generally surrounded by athermal shield. The assembly is typically contained within a vacuumenclosure.

Some superconductive magnets are conductively cooled by a cryocoolercoldhead, such as that of a conventional Gifford-McMahon cryocooler,which is mounted to the magnet. Mounting of the cryocooler coldhead tothe magnet, however, creates difficulties including the detrimentaleffects of stray magnetic fields on the coldhead motor, vibrationtransmission from the coldhead to the magnet, and temperature gradientsalong the thermal connections between the coldhead and the magnet. Suchconductive cooling is not generally suitable for cooling rotatingmagnets, such as may constitute a superconductive rotor.

Other superconductive magnets are cooled by liquid helium in directcontact with the magnet, with the liquid helium boiling off as gaseoushelium during magnet cooling and with the gaseous helium typicallyescaping from the magnet to the atmosphere. Locating the containment forthe liquid helium inside the vacuum enclosure of the magnet increasesthe size of the superconductive magnet system, which is undesirable inmany applications.

Superconducting rotors include a massive rotor core, which is typicallyat about room temperature, and a superconducting coil, which is in closeproximity to the rotor core and which must be cooled below its operatingtemperature. The presence of impurities, such as gases, in the vicinityof the superconducting coil may cause ice build-up on thesuperconducting coil. The ice build-up over time may cause rub damagebetween moving superconducting coils and may further act as a thermalshort between the rotor core and the superconducting coil(s).

Accordingly, superconducting machines, such as superconducting rotors,typically require vacuum insulation of the superconducting element(s)thereof, for example maintaining a vacuum for the superconductingcoil(s). One known solution is to warm the superconducting coil, to roomtemperature, for example, to desorb the gases, which are adsorbed on thesurface of the superconducting coil during operation thereof. However,this required shut-down and maintenance is undesirable for commercialpower generation applications.

Accordingly, it would be desirable to provide innovations in asuperconductive machine for operations over extended periods of timewithout regeneration. More particularly, it would be desirable toprevent or reduce the adsorption of gases on the superconductingelement(s) of the machine, for example on the superconducting coil(s).

SUMMARY

Briefly, in accordance with one embodiment of the invention, asuperconducting machine includes a superconductive device, and a vacuumenclosure containing and thermally insulating the superconductivedevice. A cold-trap is configured to condense gases generated within thevacuum enclosure, and a coolant circulation system is adapted to forceflow of a cryogen to and from the superconductive device and thecold-trap. A cryogenic cooling system is configured to cool the cryogenin the coolant circulation system upstream of the superconductivedevice.

A superconducting rotor embodiment includes a rotor core and at leastone superconducting coil extending around the rotor core. A vacuumenclosure 14 contains and thermally insulates the superconductive coil.A cold-trap is configured to condense gases generated within the vacuumenclosure, and a coolant circulation system is adapted to force flow ofa cryogen to and from the superconductive coil and the cold-trap. Acryogenic cooling system is configured to cool the cryogen in thecoolant circulation system upstream of the superconductive coil.

A vacuum retention method embodiment, for a high-temperaturesuperconductive HTS device, includes applying vacuum to the HTS deviceto thermally insulate the HTS device, condensing gases generated aroundthe HTS device using a cold-trap, flowing a cryogen to and from the HTSdevice, and flowing the cryogen to and from the cold-trap.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 schematically depicts a superconducting machine embodiment of theinvention;

FIG. 2 illustrates another superconducting machine embodiment of theinvention;

FIG. 3 schematically depicts a superconducting rotor embodiment of theinvention; and

FIG. 4 is a cross-sectional view of the rotor of FIG. 3.

DETAILED DESCRIPTION

As shown in FIG. 1, a superconducting machine 100 includes asuperconductive device 12. Exemplary superconducting devices 12 includesuperconducting coils and other superconducting magnets. Anotherexemplary superconducting device 12 is a high-temperature (HTS)superconducting rotor, for use in a HTS generator. A vacuum enclosure 14contains and thermally insulates the superconductive device 12. Acold-trap 60 is configured to condense gases generated within the vacuumenclosure 14. As use here, the phrase “condensing gases” includescondensation of gases and other impurities that degrade a vacuum.Cold-traps are known and used, for example, on ultra-high vacuum systemsin the semiconductor industry. For the embodiment shown in FIG. 1, acoolant circulation system 16 is adapted to force flow of a cryogen toand from the superconductive device 12 and the cold-trap 60. Exemplarycryogens include Helium, Hydrogen, Neon, Nitrogen and mixtures thereof.A cryogenic cooling system 10 is configured to cool the cryogen in thecoolant circulation system upstream of the superconductive device 12.

Cryogenic cooling systems 10 are known and will not be discussed indetail here. Exemplary cryocooling systems 10 include liquifiers,Brayton cycle refrigerators, and regenerative crycoolers. Otherexemplary crycooling systems are discussed in commonly assigned U.S.Pat. No. 6,442,949, Laskaris et al, “Cryogenic cooling refrigerationsystem and method having open-loop short term cooling for asuperconducting machine” and U.S. Pat. No. 6,438,969, Laskaris et al.,“Cryogenic cooling refrigeration system for rotor having a hightemperature super-conducting field winding and method,” which areincorporated by reference herein.

For the particular embodiment shown in FIG. 1, the coolant circulationsystem 16 includes a primary cooling line 62, which is configured tocool the superconducting device 12, and a secondary cooling line 64,which is configured to cool the cold-trap 60. Exemplary primary andsecondary cooling lines include stainless-steel pipes for flowing thecryogen.

More particularly, the coolant circulation system 16 further includes aflow control valve 66, which is configured to control and apportion theflow of the cryogen between the primary and secondary cooling lines 62,64, as indicated in FIG. 1. Alternatively, the coolant circulationsystem 16 may be designed to balance the cryogen flow resistance of theprimary and secondary cooling lines 62, 64, in order to provide thedesired distribution of cryogen flow between the primary and secondarycooling lines 62, 64.

The inlet temperature of the cryogen in primary cooling line 62 is belowthe operating temperature of the superconductive device 12. Moreparticularly, the exit temperature T2 of the cryogen in primary coolingline is about the operating temperature of the superconductive device12. As used here, the inlet and exit temperatures correspond to thetemperature of the cryogen supplied to and exiting the superconductingdevice 12, respectively, in a vicinity of the superconducting device 12.For more particular embodiments, the inlet temperature in primarycooling line 62 is at least about ten degrees Kelvin (10° K.), and moreparticularly twenty degrees Kelvin (20° K.), below the exit temperaturein primary cooling line 62.

Beneficially, the cold-trap 60 removes gases, such as Hydrogen, from thevacuum enclosure 14, in order to achieve a high level vacuum, forexample less than about 10⁻⁵ or 10⁻⁶ Torr. To more efficiently trapgases in the vacuum enclosure 14, the inlet temperature T0 for thesecondary cooling line 64 is below the triple point for Hydrogen (about13.81° K.), according to a particular embodiment. More particularly, theexit temperature T1 of the cryogen in the secondary cooling line 64 isbelow about the triple point for Hydrogen. As used here, the inlet andexit temperatures correspond to the temperature of the cryogen suppliedto and exiting the cold-trap 60, respectively, in a vicinity of thecold-trap 60. Having an inlet temperature less than the triple point forHydrogen facilitates trapping Hydrogen in the cold-trap, therebyincreasing the vacuum level within vacuum enclosure 14.

Another superconducting machine 200 embodiment is described withreference to FIG. 2. As shown, the superconducting machine 200 includesa superconductive device 12, a vacuum enclosure 14 containing andthermally insulating the superconductive device, and a cold-trap 60configured to condense gases generated within the vacuum enclosure 14. Aprimary coolant circulation system 86 is adapted to force the flow of aprimary cryogen to and from the superconductive device 12, and a primarycryogenic cooling system 88 is configured to cool the primary cryogen inthe primary coolant circulation system 88 upstream of thesuperconductive device 12. A secondary coolant circulation system 90 isadapted to force the flow of a secondary cryogen to and from thecold-trap 60, and a secondary cryogenic cooling system 92 is configuredto cool the secondary cryogen in the secondary coolant circulationsystem upstream of the cold-trap. The primary and secondary cryogens maybe the same cryogen or may comprise different cryogens, depending onsystem requirements. With the exception of the separate coolantcirculation systems and cryogenic cooling systems, the embodiment shownin FIG. 2 is similar to that depicted in FIG. 1. Accordingly, detailsdescribed above with respect to FIG. 1 will not be repeated.

According to a more particular embodiment, the superconducting machine200 further includes a rotor core 93, and the superconductive device 12includes at least one superconducting coil 12 extending around the rotorcore, as shown in FIG. 4, for example. More particularly, thesuperconducting coil 12 is a high-temperature superconducting coil. Asused here, the phrase “high-temperature superconducting” refers tomaterials that-are superconducting at temperatures of about fifteendegrees Kelvin (15° K.) or more.

A superconducting rotor 300 embodiment is described with reference toFIGS. 3 and 4. As shown in FIG. 4, the superconducting rotor 300includes a rotor core 93 and at least one superconducting coil 12extending around the rotor core. According to a particular embodiment,the superconducting coil 12 is a—high temperature superconducting (HTS)coil. As indicated in FIG. 4, a vacuum enclosure 14 contains andthermally insulates the superconductive coil 12. As illustrated in FIG.3, a cold-trap 60 is configured to condense gases generated within thevacuum enclosure 14. A coolant circulation system 16 is adapted to forcethe flow of a cryogen to and from the superconductive coil 12 and thecold-trap 60, as shown, for example in FIG. 3. A cryogenic coolingsystem 10 is configured to cool the cryogen in the coolant circulationsystem upstream of the superconductive coil 12, as indicated in FIG. 3.The coolant circulation system 16 is described above with respect toFIG. 1, and the details will not be repeated.

The rotor core 93 is typically “warm,” for example at room temperature(about 300° K.). As indicated in FIG. 4, thermal insulation 102 preventsthe rotor core 93 from warming the superconducting coil 12.

As discussed above with respect to FIGS. 1 and 2, use of the cold-trap60 to condense gases in the vacuum enclosure 14 and, more particularly,maintaining the inlet temperature T0 for the cryogen flowing tocold-trap 60 below the triple point of Hydrogen facilitates achieving ahigh level of vacuum within vacuum enclosure 14.

A vacuum retention method for a high-temperature superconductive (HTS)device 12 is also disclosed. The method includes applying vacuum to theHTS device 12 to thermally insulate the HTS device, condensing gasesgenerated around the HTS device using a cold-trap 60, flowing a cryogento and from the HTS device to cool the HTS device, and flowing thecryogen to and from the cold-trap. More particularly, the method furtherincludes cooling the cryogen upstream of the HTS device 12 andcontrolling the flow of the cryogen to the HTS device 12 and thecold-trap 60. Still more particularly, the method further includestrapping the condensed gases using the cold-trap 60.

As noted above with respect to the machine embodiment, the inlettemperature of the cryogen flowing to the HTS device 12 is below anoperating temperature of the HTS device. More particularly, the exittemperature of the cryogen flowing from the HTS device is about theoperating temperature of the HTS device. To enhance trapping within thecold-trap 60 and thereby increase the level of vacuum for the HTS device12, the inlet temperature of the cryogen flowing to the cold-trap 60 isbelow the triple point for Hydrogen. More particularly, the exittemperature of the cryogen flowing from the cold-trap 60 is below aboutthe triple point for Hydrogen.

Although only certain features of the invention have been illustratedand described herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A superconducting machine comprising: a superconductive device; avacuum enclosure containing and thermally insulating saidsuperconductive device; a cold-trap configured to condense gasesgenerated within said vacuum enclosure; a primary coolant circulationsystem adapted to force flow of a primary cryogen to and from saidsuperconductive device, wherein said primary coolant circulation systemcomprises a primary cooling line configured to cool said superconductingdevice; a primary cryogenic cooling system configured to cool theprimary cryogen in said primary coolant circulation system upstream ofsaid superconductive device; a secondary coolant circulation systemadapted to force flow of a secondary cryogen to and from said cold-trap,wherein said secondary coolant circulation system comprises a secondarycooling line configured to cool said cold-trap, wherein said secondarycoolant circulation system does not supply the secondary cryogen to saidprimary coolant circulation system, and wherein an inlet temperature ofsaid secondary cooling line is below about the triple point forHydrogen; and a secondary cryogenic cooling system configured to coolthe secondary cryogen in said secondary coolant circulation systemupstream of said cold-trap.
 2. The superconducting machine of claim 1,further comprising a rotor core, wherein said superconductive devicecomprises at least one superconducting coil extending around said rotorcore.
 3. The superconducting machine of claim 2, wherein saidsuperconducting coil comprises a high-temperature superconducting coil.